The invention relates to a circuit that supplies a pair of voltages particularly suitable for operating emitter coupled logic (ECL) circuitry. One voltage typically biases the ECL current sources and is referenced against V.sub.EE, the negative power supply terminal. The other voltage is used as the switching threshold level and is referenced against the positive power supply terminal, V.sub.CC. As a practical matter ECL circuits have relatively small voltage swings and must operate in the presence of variable supply voltages and noise that is certain to be present to some degree. One measure of the excellence of an ECL system is its noise immunity and tolerance to supply voltage variations. In one sense the ideal system would provide the two voltages referenced respectively to the + and - supply terminals in such a manner that they do not vary with either loading conditions or temperature. Additionally they will not vary with variations in the power supply voltage. As a practical matter these conditions can be approached. If desired, the two voltages can be made to have a controlled temperature that is tailored to suit the ECL system requirement.
U.S. Pat. No. 3,893,018 to Robert R. Marley shows an approach to ECL power supplies and one embodiment will be described in detail hereinafter. While the circuit provides useful ECL operation, the noise margins and supply variation tolerance leave considerable room for improvement.